Refactor sampleTexel() to use a return value.

Refactor sampleTexel() to use a return value.

We can rely on return value optimization to return Reactor types at the
same cost as passing them by reference.

Change-Id: I9f092bcefd17131a07bef32032c5f2ab98392328
Reviewed-on: https://swiftshader-review.googlesource.com/10569
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>